Toggle the Object keys boolean values, setting all to false, but the key you want to set as true.

truthify

function truthify (scope, child) {

	var obj = scope;
	
	// set param to true on selectee
	obj[child] = true;
	
	for (var el in obj) {
		
		// if obj is set to true && not equal to who
		if(obj[el] && el != child){
			// reverse bool
			obj[el] = !obj[el];
		}
	}
	// console.log(obj); // see output change
};

// Usage Example
var obj = {
	"level" : {
		"ace" : true,
		"base": false,
		"case": false
	}
};
// set 'base' to true, and rest as false
truthify(obj.level, 'base');